I have a BTAS 410 regged gun, and three other non regged 400 series and to be honest I wouldn't bother.
My main HFT competition gun is an non regged MPR which shoots extremely well and with which I achieved 2nd overall in the NEFTA open class last year. I am sure non regged 400's have won their fair share of titles, and its something else to go wrong and be affected with temperature changes, or extremes.
Tench and Tweeky are the same person, and I have previously bought and tried a Czech reg and sent it back as it didn't work like a reg, morelike a bigger firing pot, although I think he has changed the design since.

I don't know much about nick murphy or similar so I cannot comment on those.
I must admit that my BTAS regged 410 has been very good, but I don't think that I would go that route myself it hadn't been fitted already.

Tench and Rob Lane probably have the best regs for the 400's if you do go down that route.

Robert lane does them for £90 on ebay with full video guides on how they work, how to service them, how to fit them and all that. They are very good and If i was in the market for a reg he would be my choice.